How to modify the version number of self-build source packages?

[rrp <rrp@gmx.net>]

Hello List,

today I successfully built a Debian source package for the first time.

That source package is 'gcc-3.0' from woody (3.0 r0), and my installed
Debian system is also woody.

So now I have two binaries for gcc-3.0: The original
gcc-3.0_3.0.4-7_i386.deb and my selfmade gcc-3.0_3.0.4-7_i686.deb.

To distinguish between these packages and to persuade apt-get and dpkg
that my *i686.deb is newer, I would like to alter the version number
during the build of the source package.
Originally it is '3.0.4-7' and I would like to have it a higher version,
for example '3.0.4-7.686' (please correct me if that is not higher than
3.0.4-7).

I use these steps to build a Debian source package (as written in the
official FAQ):

     dpkg-source -x foo_version-revision.dsc
     debian/rules build
     debian/rules binary

Question:
How can I build my source package with an altered version number?

Many thanks in advance,
Jan